RFR: 8210061: ZGC: Remove STW weak processor mode

[Erik Österlund]

Hi,

ZGC has multiple modes for processing weaks. ZWeakRoots allows treating 
weaks as strong, and in addition to that, ZConcurrentStringTable, 
ZConcurrentVMWeakHandles and ZConcurrentJNIWeakGlobalHandles allow 
certain subsets of weak roots to be processed concurrently or in a 
pause. Except there is no obvious reason why you would ever opt out of 
doing it concurrently. So I propose to remove this functionality.
Part of the motivation is that a concurrent strong root processor is on 
its way, and all these modes nobody uses leads to pointless complexity.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8210061/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8210061

Thanks,
/Erik